Synoptic Regimes Research Articles

Wintertime Synoptic Patterns of Midlatitude Boundary Layer Clouds Over the Western North Atlantic: Climatology and Insights From In Situ ACTIVATE Observations

AbstractThe winter synoptic evolution of the western North Atlantic and its influence on the atmospheric boundary layer is described by means of a regime classification based on Self‐Organizing Maps applied to 12 years of data (2009–2020). The regimes are classified into categories according to daily 600‐hPa geopotential height: dominant ridge, trough to ridge eastward transition (trough‐ridge), dominant trough, and ridge to trough eastward transition (ridge–trough). A fifth synoptic regime resembles the winter climatological mean. Coherent changes in sea‐level pressure and large‐scale winds are in concert with the synoptic regimes: (a) the ridge regime is associated with a well‐developed anticyclone; (b) the trough‐ridge gives rise to a low‐pressure center over the ocean, ascents, and northerly winds over the coastal zone; (c) trough is associated with the eastward displacement of a cyclone, coastal subsidence, and northerly winds, all representative characteristics of cold‐air outbreaks; and (d) the ridge–trough regime features the development of an anticyclone and weak coastal winds. Low clouds are characteristic of the trough regime, with both trough and trough–ridge featuring synoptic maxima in cloud droplet number concentration (Nd). The Nd increase is primarily observed near the coast, concomitant with strong surface heat fluxes exceeding by more than 400 W m−2 compared to fluxes further east. Five consecutive days of aircraft observations collected during the ACTIVATE campaign corroborates the climatological characterization, confirming the occurrence of high Nd for days identified as trough. This study emphasizes the role of boundary‐layer dynamics and aerosol activation and their roles in modulating cloud microphysics.

High Spatiotemporal Resolution Planetary Boundary Layer Dynamics Across the Israeli Coast—Mountain—Valley Terrain Unraveled by WRF Simulations

AbstractThe west‐east terrain cross‐section along ∼150 km of the northern part of Israel has a coastal plain—mountain—valley structure; hereafter denoted iCMV. The boundary layer height (BLH) evolution mechanism across the iCMV has not yet been fully understood. We use Weather Research and Forecasting (WRF) simulations and ceilometer measurements to decipher the iCMV BLH evolution mechanism during the late summertime period of 5–14 September 2017, where the daily maximum BLH at the mountainous city of Jerusalem (JrM; 800 m above sea level) varies by ∼1,000 m. We first verify the BLH, simulated by five model configurations (with four different BL schemes). The RMSE for the three best configurations are around 160 and 200 m for the coastal and JrM areas, respectively. An analysis of the simulated daily BL evolution reveals a general mechanism. In the early morning, the upslope flows and synoptic forcing interact to induce a surface flow convergence (SFC) zone. During a westerly (easterly) synoptic flow, the SFC is induced east (west) of JrM. Assisted by the inland propagating sea breeze front, the SFC is advected eastwards during the afternoon, until it collapses at the valley bottom. The SFC zone is accompanied by a substantial positive vertical wind column and a locally elevated BLH. The more western the morning time SFC is, the more likely the elevated BLH will pass during midday through JrM, with a higher daily maximum BLH. The exact dependence of the BLH evolution and of the associated temperature and humidity variations upon the synoptic regime require further study.

Is the Monsoon climatology observed over the National Capital Region, Delhi indicative of an Urban -Monsoon linkage on rainfall modification?

The Indian Summer Monsoon (ISM) plays a vital role in the lives of the National Capital Region (NCR) - Delhi, the second biggest urban settlement globally. This study attempt to assess the spatio-temporal characters and decadal changes of monsoonal features in detail. The climatology of Monsoonal rainfall and Monsoon Low Level Jet (MLLJ) are investigated. NCR Delhi exhibits a ∼ 28.5% in the recent decade and ∼ 19.7% in the 1997–06 decade reduction in rainfall is noticed compared with 1987–96 decade. The study reveals that the precipitation reduction is alongside the weakening of the MLLJ and marginal reduction of core height over the region. Also, the magnitude of surface wind declined with the directional shift of both MLLJ and surface winds notable in southerlies in the 2007–16 decade. Rainfall islands parallel to the urban heat islands (UHI) are seen likely as coupled monsoon-urban induced effects under the weakened synoptic regime evidenced through MLLJ. The Land Surface Temperature difference (UHI) of ∼ 2.5 °C and more than 3.5 °C is observed from the urban centre to the surrounding cropland during the months of August (core monsoon) and September, respectively. In addition to the rainfall islands during August, islands of rainfall in both the upwind and downwind directions of the urban centre are noted in September. The rainfall patterns point out the signal of monsoon-rainfall modification due to NCR urbanization. Further, the influence of crosswinds with Aravali ridges and the presence of Himalayan foothills are also notable in the NCR rainfall patterns.

The Impact of Topography on the Environment and Life Cycle of Weakly and Strongly Forced MCSs during RELAMPAGO

Abstract Intense deep convection and large mesoscale convective systems (MCSs) are known to occur downstream of the Andes in subtropical South America. Deep convection is often focused along the Sierras de Córdoba (SDC) in the afternoon and then rapidly grows upscale and moves to the east overnight. However, how the Andes and SDC impact the life cycle of MCSs under varying synoptic conditions is not well understood. Two sets of terrain-modification experiments using WRF are used to investigate the impact of topography in different synoptic regimes. The first set is run on the 13–14 December 2018 MCS case from RELAMPAGO, which featured a deep synoptic trough, strong lee cyclogenesis near the SDC, an enhanced low-level jet, and rapid upscale growth of an MCS. When the Andes are reduced by 50%, the lee cyclone and low-level jet that develop are weaker than with the full Andes, and the resulting MCS is weak and moves faster to the east. When the SDC are removed, few differences between the environment and resulting MCS relative to the control run are seen. A second set of experiments are run on the 25–26 January 2019 case in which a large MCS developed over the SDC and remained tied there for an extended period under weak synoptic forcing. The experiment that produces the most similar MCS to the control is when the Andes are reduced by 50% while maintaining the height of the SDC, suggesting the SDC may play a more important role in the MCS life cycle under quiescent synoptic conditions.

Bores Observed During the Warm Season of 2015–2019 Over the Southern North China Plain

AbstractThis first multi‐year investigation focuses on bores over the southern North China Plain during the 2015–2019 warm seasons. Bore structure depended on location with undular bores tending to occur close to the coast and non‐undular bores to the west near elevated terrain. Bores were most likely to occur during June and July when convection is active. While bore frequency over the Southern Great Plains (SGP) of U.S. is linked to the region's nocturnal low‐level jet, the bores herein were sensitive to the synoptic regime with ∼80% occurring during 4‐to‐5‐day periods under three different synoptic regimes. The bores had shorter durations than their SGP counterparts and a far wider range in their direction of propagation. Overall, these findings find regional differences in bores' frequency, movement, and structure serving an impetus for future investigations of nocturnal mesoscale convective systems and bores over China and other locations worldwide.

A 25-year climatology of low-tropospheric temperature and humidity inversions for contrasting synoptic regimes at Neumayer Station, Antarctica.

A 25-year set of daily radiosonde data was used to investigate temperature and humidity inversions at Neumayer Station, coastal Dronning Maud Land, Antarctica. For the first time, inversions were studied differentiating between different synoptic conditions and different height levels. It was shown that, generally, inversions occurred on the majority (78%) of the days, with simultaneous occurrence of humidity and temperature inversions being observed on approximately two thirds of all days. Multiple inversions are common in all seasons for cyclonic and noncyclonic conditions, however, typically occur more frequently under cyclonic conditions. The seasonality of inversion occurrence and features, that is, inversion strength, depth and vertical gradients, was analysed statistically. Different formation mechanisms depending on inversion levels and prevailing weather situations are related to typical annual courses of certain inversion features. Winter maxima were found for the features that are mostly connected to the temperature close to the surface, which is mainly a result of the negative energy balance, thus influencing surface-based inversions. At the second level, both temperature and humidity inversions are often caused by advection of comparably warm and moist air masses related to the passage of cyclones and their frontal systems. Hence, maxima in several inversion features are found in spring and fall, when cyclonic activity is strongest. Monthly mean profiles of humidity and temperature inversions reveal that elevated inversions are often obscured in average profiles due to large variations in inversion height and depth.

Retrospective sub-seasonal forecasts of extreme precipitation events in the Arabian Peninsula using convective-permitting modeling

This work demonstrates the potential of extreme cool-season precipitation forecasts in the Arabian Peninsula (AP) at sub-seasonal time scales, identifies regions and periods of forecast opportunity, and investigates the predictability of synoptic-scale forcing at sub-seasonal time scales. To this end, we simulate 18 extreme precipitation events using the convective-permitting Weather Research and Forecasting (CP-WRF) model with lateral boundary forcing from the European Centre of Medium-range Weather Forecasts sub-seasonal to seasonal reforecasts (ECMWF S2S reforecasts). The simulations are initiated at one-, two-, and three-week lead times. At all lead times, the CP-WRF improves the mean accumulated precipitation in the extratropical synoptic regimes over the west coastal and central AP and the central Red Sea compared to ECMWF S2S reforecasts as evaluated against the Global Precipitation Measurement Final (GPMF) and King Abdullah University of Science and Technology reanalysis (KAUST-RA) precipitation products. Based on categorical statistics with a threshold of 20 mm accumulated precipitation over 7 days, the CP-WRF skillfully forecasts the precipitation over Jeddah, the west coast of AP, and the central Red Sea up to three-week lead time. The relative operating characteristic curve reconfirmed the high forecast skill of the CP-WRF, with an area under the curve above 0.5 in most of the events at all lead times. Finally, the correlation coefficients between the ECMWF S2S reforecast and ECMWF reanalysis interim 500 hPa geopotential heights are higher in the events associated with an extratropical synoptic regime than in those associated with a tropical synoptic regime, regardless of lead time. Therefore, the convective-permitting model can potentially improve the accuracy of extreme winter precipitation forecasts at two-and three-week lead times over Jeddah, the west coast of AP, and the central Red Sea in the extratropical synoptic regime.

Multiscale Interactions Contributing to Enhanced Orographic Precipitation in Landfalling Frontal Systems over the Olympic Peninsula

Abstract Orographic precipitation results from complex interactions between terrain, large-scale flow, turbulent motions, and microphysical processes. This study appeals to polarimetric radar data in conjunction with surface-based disdrometer observations, airborne particle probes, and reanalysis data to study these processes and their interactions as observed during the Olympic Mountain Experiment (OLYMPEX). Radar and disdrometer observations from OLYMPEX, which was conducted over the Olympic Peninsula in the winter of 2015, revealed 3 times as much rain fell over elevated sites compared to those along the ocean and coast. Several events were marked by significant water vapor transport and strong onshore flow. Detailed analysis of four cases demonstrated that the warm sector, which previous authors noted to be a period of strong orographic enhancement over the terrain, is associated not only with deeper warm cloud regions, but also deeper cold cloud regions, with the latter supporting the growth of dendritic ice crystals between 4 and 6 km. This dendritic growth promotes enhanced aggregation just above the melting layer, which then seeds the warm cloud layer below, allowing additional drop growth via coalescence. Periods of subsynoptic variability associated with mesoscale boundaries and low-level jets are shown to locally modify both the ice microphysics as well as surface drop-size distributions. This study explores the spatial and temporal variability of precipitation, cloud microphysics, and their relationship over the complex terrain of the Olympic Peninsula. Significance Statement This study appeals to polarimetric radar, aircraft particle probes, disdrometer data, and reanalysis to investigate the complex interactions between large frontal systems, terrain, and microphysical processes contributing to precipitation characteristics at the surface over the Olympic Peninsula. The study finds that the precipitation is a complex function of the synoptic regime, distance inland, and terrain height. Ice microphysical processes aloft act to modulate the surface rain drop size distributions, and are more important in contributing to higher rain accumulations inland during the later phases of the warm sector, particularly over the middle terrain heights (100–500 m).

Linking Synoptic Patterns to Cloud Properties and Local Circulations Over Southeastern Texas

AbstractThis study classifies meteorological regimes in the southeastern Texas region to identify environmental conditions that favor sea‐breeze induced convection. The classification is accomplished using a Self‐Organizing Map (SOM) approach. We applied SOM to 10 years of 700‐hPa geopotential height anomalies during the summer months from reanalysis data to distinguish three dominant synoptic regimes, with a continuum of transitional states between those. The primary regimes include: (a) a pre‐trough regime associated with a synoptic trough, (b) a post‐trough regime with upper‐level northerly flow, and (c) an anticyclonic regime within the westward extent of the Bermuda High. We project the data from the Geostationary Operational Environmental Satellite and the Next Generation Weather Radar system onto each SOM node to investigate the characteristics of cloud and precipitation properties in different regimes. When southeastern Texas is positioned to the southwest quadrant of a maritime high pressure system, an increased cloud frequency is observed over the region during the afternoon hours due to significant moisture advection. A confluence of synoptic southerly flow and sea‐breeze circulation commonly occurs in this regime. When a high pressure system is over southeastern Texas, the area is dominated by large‐scale subsidence with weak pressure gradients and moderate precipitable water vapor. This weak synoptic forcing is favorable for the formation of a sea‐breeze circulation. This is confirmed by an enhanced onshore flow and a decreased temperature at the surface in the early afternoon, as well as a sharp increase in radar echo top height.

Dynamical and thermodynamical interactions in daily precipitation regimes in the Western Himalayas

AbstractThe mechanisms by which moisture interacts with the Himalayas largely determine the amount of precipitation in Himalayan basins. While many recent studies have focused on mechanisms of independent precipitation events, climatological studies that are essential for a complete understanding of precipitation‐generating mechanisms are limited. This work presents synoptic regimes, which produced precipitation across all seasons in the Western Himalayas (WH) from 2000 to 2018. Using the k‐means clustering algorithm, seven clusters are employed to define relatively mild, moderate and wet regimes, showing distinct seasonality and a synoptic meteorology. We found positive precipitation anomalies at lower elevations in monsoonal regimes (M1, M2 and M3) but at higher elevations in winter (W1 and W2) and transitional regimes (T1 and T2). Moist monsoonal regimes are associated with dynamical interactions between low‐level tropical cyclonic circulations and mid‐level subtropical troughs. Synchronous primary and secondary cyclonic circulations facilitate tropical moisture influx and obstruct the further northward movement of cyclonic circulations, which results in large magnitudes of precipitation at lower elevations in monsoonal regimes. On the other hand, winter regimes exhibit intense western disturbances, which enable orographic ascent of tropical moisture towards higher elevations. Despite weaker dynamical interactions, a stronger thermodynamical instability and a steeper terrain gradient trigger deep convection at higher elevations in transitional regimes. Overall, monsoonal regimes account for 52% of rainy days, whereas winter and transitional regimes account for 20 and 28%, respectively. We present a methodology that identifies hotspots of anomalous precipitation over vulnerable higher elevations by tracking atmospheric variables in Delhi. Our results illustrate the dynamical and thermodynamical interactions responsible for precipitation and highlight the significant contribution from nonmonsoonal regimes to the precipitation across higher elevations in the WH.

Self‐organizing map classification of the boundary layer profile: A refinement of Eastern Mediterranean winter synoptic regimes

AbstractThe boundary layer (BL) profile over the coastal plain of Israel, Eastern Mediterranean (EM), varies considerably during winter. Although in the context of air pollution, the characteristics of the BL height (BLH) was intensively investigated, a quantitative classification of the BL profile regimes has not been performed. Here, we seek to reveal the dominant, recurring regimes of the BL profiles, their quantitative characteristics and links to regional synoptic‐scale patterns. An objective unsupervised classification of winter BL radiosonde profiles is performed for the first time by multi‐parameter self‐organizing map (SOM) analysis. The analysis uses high‐resolution, 12:00‐UTC data of wind, temperature, humidity and pressure measurements during December to February 2007–2018, and yields 30 distinct profile regimes. Composite analysis using ERA5 reanalysis suggests strong association between the profile regimes and synoptic weather systems and highlights four groups: (a) Deep winter cyclones with strong westerly wind and precipitation; (b) Strong surface anticyclones and Red Sea troughs (RST) with a mid‐tropospheric ridge, moderate dry easterly wind and extreme temperatures. (c) Moderate pressure gradients under shallow cyclones, anticyclone to the west and RST to the east of Israel. (d) Active RSTs, accompanied by upper‐tropospheric trough/cutoff low and heavy precipitation. For the first time, general objective classification observes the active RST without requiring specific criteria. Consistent with previous knowledge, the new classification exhibits distinct categories of thermal stability, BLH and turbulence. Importantly, we show that the automatic objective classification of profile data from a single station can be a sensitive discriminator of winter synoptic regimes in the EM, and therefore explains the variability of the BL profile. It facilitates the study of the interaction between the BL and the free troposphere and may improve the prediction of air pollution or future BL profile regimes based on long time series from historical data or climate models.

Dependence of predictability of precipitation in the northwestern Mediterranean coastal region on the strength of synoptic control

Abstract. The weather-regime-dependent predictability of precipitation in the convection-permitting kilometric-scale AROME-EPS is examined for the entire HyMeX-SOP1 employing the convective adjustment timescale. This diagnostic quantifies variations in synoptic forcing on precipitation and is associated with different precipitation characteristics, forecast skill and predictability. During strong synoptic control, which dominates the weather on 80 % of the days in the 2-month period, the domain-integrated precipitation predictability assessed with the normalized ensemble standard deviation is above average, the wet bias is smaller and the forecast quality is generally better. In contrast, the pure spatial forecast quality of the most intense precipitation in the afternoon, as quantified with its 95th percentile, is superior during weakly forced synoptic regimes. The study also considers a prominent heavy-precipitation event that occurred during the NAWDEX field campaign in the same region, and the predictability during this event is compared with the events that occurred during HyMeX. It is shown that the unconditional evaluation of precipitation widely parallels the strongly forced weather type evaluation and obscures forecast model characteristics typical for weak control.

Characteristics of temperature inversion from radiosonde measurements in the Western Ghats region

The characteristics of temperature inversion (TI) are believed to be dependent on topographical features and synoptic regime. Western Ghats (WGs) mountains located in the west coast of India have a complex topography, and the TI features for this region remain unknown due to the complexity and non-availability of observational datasets. This study investigates TI characteristics in the troposphere using high-resolution radiosonde data (2012–2015). The radiosonde measurements are collected at Mahabaleshwar (17.92°N, 73.6°E, ~1.4 km above mean sea level, AMSL), a high-altitude site in the WGs. The characteristics of TI in terms of height, depth, and strength (i.e., temperature difference between base and top of the inversion layer) are studied. The results showed that TI occurs in all seasons, however, their frequency differs. Winter (Dec.-Feb.) shows the maximum number of inversions followed by pre-monsoon (Mar.-May), post-monsoon (Oct.-Nov.), and monsoon (Jun.-Sep.). In winter, the depth of inversion is higher; however, the inversion strength is weaker as compared to other seasons. The TI strength is always below 5 K. Apart from surface-based inversions, elevated and upper-tropospheric inversions are also observed. Vertical and horizontal wind shear profiles along with ERA-5 data were analyzed to examine the plausible mechanism for TI to occur.

Deforestation Impacts on Orographic Precipitation in the Tropical Andes

This study examines the implications of Tropical Montane Forests (TMFs) loss on orographic precipitation in the Eastern slopes of the Andes (EADS). The focus is on moist processes for synoptic regimes associated with significant EADS precipitation: (1) monsoon rainfall for weak and strong South America Low-Level Jet (LLJ) conditions and (2) heavy rainfall associated with cold air intrusions (CAI) in the dry season. High-resolution simulations using the Weather Research and Forecasting (WRF) model were conducted for realistic and modified land-cover resulting from the conversion of TMFs to savanna. The deforestation scenarios result in 50–100% decrease (up to ∼400 J kg–1) in Convective Available Potential Energy (CAPE) spatially organized by land-cover change along the EADS. Analysis of the differences in simulated frequency distributions of rainfall intensity shows robust daytime increases in light rainfall (<2 mm h–1) and decreases in moderate rainfall rates (2–10 mm h–1) in the altitudinal band 500–2,000 m where orographic enhancement is dominant. Whereas there are negligible changes in the spatial patterns of precipitation and hydrologic response for monsoon conditions, rainfall accumulations decrease for all cases, and the precipitation maxima shift downslope into the Amazon lowlands. Changes in rainfall amount and intensity result in runoff decreases of 5–10% at the event-scale for the CAI case. Sensitivity simulations for lower initial soil moisture conditions indicate a strong positive feedback of forest loss to hydrologic drought along the EADS foothills in the austral spring when CAIs play a key role in the tropical EADS dry season hydrometeorology.

Aerosol pattern changes over the dead sea from west to east - Using high-resolution satellite data

The area of the Dead Sea, a terminal lake on the border between Israel and Jordan, has gone through extensive environmental changes in recent decades, stemming from several processes including extreme evaporation and land degradation. Our study explored the distribution and the long-term high-resolution climatology of the Dead Sea aerosols, using satellite-retrieved Aerosol Optical Depth (AOD) measurements. We used the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm that provides AOD measurements at a 1 km resolution. MAIAC retrievals for the entire study period (2000–2016) have shown a very good agreement (r = 0.8) with the nearby AERONET station. For the first time, the AOD temporal distribution and its variability, on both Dead Sea coasts, were calculated on a monthly/seasonal basis. Three main aerosol pollution patterns (i.e. high-levels of AOD) emerged as follows: western focalized (75 and 59% of the months examined for Aqua and Terra, respectively), eastern focalized (14 and 24%), and similar levels of pollution on both coasts (11 and 17%, respectively). Using the Normalized AOD Difference (NAODD) metric, we studied how the AOD spatial patterns changed and when. Following the negative trends in most of the months, our analysis has demonstrated a shift towards unexplained increasing pollution levels over the Jordanian eastern side. For March–April, this shift had already occurred, and the potential reasons including changes in synoptic regimes are discussed. Local and global implications are of much regional interest because of the Dead Sea deterioration and the need for urgent environmental policy changes.

Tornadoes in Portugal: An Overview

From 2000 onwards, a systematic documentation of tornado reports was maintained at the Portuguese Meteorological Service (IPMA). The characteristics of 195 tornado events over land and at sea off the coast of mainland Portugal, Azores, and Madeira Islands, reported until 2020, were compiled into a new tornado database for Portugal. Each event was identified through the direct observation of the vortex, photography or video footage, or eyewitness descriptions, as well as by site surveys, including the interpretation of traces of the tornado found on the ground. For events after 2006, each data record was complemented with Doppler radar observations and derived products, which allowed a tornado type (TT) classification. The synoptic regimes and atmospheric environments favoring each type were identified using observations and numerical weather prediction model data. Results showed that tornadoes over Portugal were more frequent during autumn, winter, and spring. It was found that the occurrence of more than one tornado on the same day was frequent. The most intense tornadoes, classified as F3 intensity, were spawned by supercells, but a large proportion of weaker, shorter-lived, but still damaging tornadoes were found to be spawned in association with quasi linear convective systems (QLCS).

Projection and possible causes of summer precipitation in eastern China using self-organizing map

Self-organizing map (SOM) is used to simulate summer daily precipitation over the Yangtze–Huaihe river basin in Eastern China, including future projections. SOM shows good behaviors in terms of probability distribution of daily rainfall and spatial distribution of rainfall indices, as well as consistency of multi-model simulations. Under RCP4.5 Scenario, daily rainfall at most sites (63%) is projected to shift towards larger values. For the early 21st century (2016–2035), precipitation in the central basin increases, yet decreases occur over the middle reaches of the Yangtze River as well as a part of its southeast area. For the late 21st century (2081–2100), the mean precipitation and extreme indices experience an overall increase except for a few southeast stations. The total precipitation in the lower reaches of the Yangtze River and in its south area is projected to increase from 7% at 1.5 °C global warming to 11% at 2 °C, while the intensity enhancement is more significant in southern and western sites of the domain. A clustering allows to regroup all SOM nodes into four distinct regimes. Such regional synoptic regimes show remarkable stability for future climate. The overall intensification of precipitation in future climate is linked to the occurrence-frequency rise of a wet regime which brings longitudinally closer the South Asia High (eastward extended) and the Western Pacific Subtropical High (westward extended), as well as the reduction of a dry pattern which makes the two atmospheric centers of action move away from each other.

Main atmospheric regimes of the long winter Red Sea trough

In this study, the Sudan low and the Red Sea trough (RST) are objectively detected using the mean sea level pressure data from the NCEP/NCAR reanalysis data set. The results show that the RST exists for 37.45% of winter and that approximately 64% of RSTs are classified as long RSTs. Most of these long RSTs are generated at night and decrease in abundance from December to February; thus, the generation timing is affected by high water-land temperature contrasts. Additionally, this study demonstrates that the Sudan low predominantly forms over the South Sudan region (70.59%) and to a lesser extent over the Sudan region (26.06%). The empirical orthogonal function (EOF) statistical analysis of long RST events indicates that the first ten modes represent 82% of the cases and that the first four modes represent 57.86% of the cases. For simplicity, this synoptic study focuses on the first four modes because they reliably represent the long RST modes. The synoptic study of the four modes demonstrates that as the mode percentage increases, the main dominant synoptic regimes that are associated with the modes and that affect the Red Sea region decrease in number and intensify in strength. Thus, the highest percentage modes are those with the lowest numbers, and strong atmospheric systems are distributed over large areas of the northern Red Sea. Furthermore, this study shows that the relative positions of the main synoptic components consisting of the northern European low-pressure system/Azores high-pressure system or the Siberian high-pressure system cause the long RSTs to be oriented east of or over the Red Sea, respectively. In addition, the study shows that the maximum wind core area expands eastward under strong mode conditions and shrinks westward from the Red Sea under weak mode conditions.

Convective Precipitation Efficiency Observed in the Tropics

AbstractPrecipitation efficiency refers to the fraction of condensate in the atmosphere that reaches the surface as precipitation. A high‐quality data set of radar‐estimated precipitation rates and convective scale vertical velocity near Darwin, Australia, is used to construct the first estimate of precipitation efficiency at convective scales for a long record of observations in the tropics. It is found that precipitation efficiency increases with precipitation rate and midtropospheric humidity and decreases with increasing convective available potential energy and surface temperature. Precipitation efficiency is largest under moist monsoonal conditions and smallest during monsoon break periods, which are characterized by a drier free troposphere. However, these differences in efficiency do not translate to differences in the instantaneous precipitation rate across the synoptic regimes because of a compensating change in the net condensation rate. This is driven by variations in cloud updraft velocity, which is larger in drier environments than in moist environments.

Analyses of the winter low‐level jet over the southern Red Sea using the Weather Research and Forecasting model

AbstractThe three‐dimensional structure of the Red Sea Low‐Level Jet (RSLLJ) that occurs over the southern Red Sea during the winter season is examined using the Weather Research and Forecasting (WRF) model. The causes of the jet formation are investigated, and the simulations confirm that the jet is a terrain‐induced phenomenon. It initiates as a gap flow as the jet forms north of the strait of Bab el‐Mandab as a result of a hydraulic effect when a stably stratified cool layer is channeled through the mountainous gap at low levels over the strait into the southern Red Sea. The jet intensity is enhanced by the Red Sea trough over the southern Red Sea. The cool dense air mass south of the strait is supported by a generally persistent synoptic regime that includes a high‐pressure system over most of the eastern Arabian Peninsula and the Sudan low‐pressure system over Sudan. The jet extends more than 500 km northward to slightly north of 17°N from the strait around 12.5°N, with a width of 100–150 km over the southern Red Sea. The jet core lies at 500–600 m above sea level between 14°N and 16°N, with wind speeds exceeding 25 m/s. The north–south pressure gradient along the strait, as well as the depth of the stratified cool layer south of the strait, are directly responsible for the jet length and speed over the southern Red Sea, with stronger pressure gradients and deeper cool layers resulting in longer and stronger jets. Sea/land breezes also directly influence the spatial scale, intensity, and diurnal variation of the jet, with land breezes causing faster and more constricted jets at night and divergence due to sea breezes leading to relatively slower jets during the daytime.